1. Technical Field
The present invention relates to image processing apparatus having an automatic document feeder.
2. Description of the Related Art
In an image processing apparatus that carries out transmission or printing of images read out from an original document, very often an automatic document feeder is installed which issues document sheets one sheet at a time from a stack of document sheets placed on the document loading tray and conveys them to the document reading position. In such image processing apparatuses, since there is no need for the user to place each sheet of the document manually at the reading position, it is possible to complete the reading of the stack of document sheets automatically even if the user is not present near the apparatus.
However, when the automatic document feeder feeds document sheets one sheet at a time from a stack of document sheets placed on the document loading tray, because of the friction force acting between document sheets, sometimes multiple feeding is made with a plurality of document sheets being conveyed simultaneously. When multiple feeding occurs, it is possible to read only the image of the document sheet that is facing the reading surface and the image of the other document sheets being transported simultaneously will be missing.
In addition, even when multiple feeding occurs, if the document conveying is continued as such until all the sheets in the document sheet stack on the loading tray have been exhausted, the user may not become aware that the read out document is not complete, or even if the user becomes aware, it was a very tedious work to confirm which are the sheets whose images are missing.
In view of this, a technology has been proposed of detecting the occurrence of multiple feeding by measuring the intensity of light passing through the document or by measuring the force of the document acting on the conveyor belt of the automatic document feeder, stopping the conveying of document sheets at the point of time when the multiple feeding occurred so that it is possible to identify the page of the document at which the multiple feeding occurred, and posting this information to the user (see, for example, Patent Document 1).
Patent Document 1: Japanese Unexamined Patent Application Open to Public Inspection No. 7-315634
In the methods of detected multiple feeding by measuring the amount of transmitted light or by measuring the pressure acting on the conveyor belt, since only the occurrence of multiple feeding is detected, in order for the user to recognize at which page did the multiple feeding occurred, it is unavoidable to stop conveying the document at the instant when the multiple feeding occurred.
However, if the conveying of document is stopped at the instant when a multiple feeding occurred, there were that problems that, if the user requesting that job is not present in the vicinity of the apparatus or is in a condition in which the user cannot receive the message indicating multiple feeding, the automatic document feeder would remain unattended in the stopped condition thereby causing inconvenience to the other users waiting to use the apparatus.
In addition, in the method of detecting the occurrence of multiple feeding by measuring the amount of transmitted light or by measuring the pressure acting on the conveyor belt, there may be the possibility of erroneous detection due to changes in the paper quality of the document.